The present invention is directed to a method and apparatus for electrically achieving sustained respiration (ventilation) in a patient. The method is particularly applicable in cases of respiratory paralysis from injury of the spinal cord at the cervical level, but should not be solely limited to such an application. Intermittent electrical stimulation is applied to the phrenic nerves and spinal cord in such a manner as to produce coordinated contraction and relaxation of the diaphragm and the inspiratory intercostal muscles. This results in the restoration of respiration rhythmically through artificial electrical stimulation.
In this regard, normal respiration relates to the act of breathing, i.e., inhaling (inspiration) and exhaling (expiration). It is by the act of breathing that the lungs are supplied with oxygen and carbon dioxide is removed during exhalation.
Specifically, during inspiration, air is inhaled into the lungs and is transferred to the blood by the gaseous exchange that occurs by the capillaries in the walls in the pulmonary alveoli. The oxygen present in the blood is utilized by the tissues resulting in the production of carbon dioxide. The carbon dioxide is in turn removed from the blood by a similar gaseous exchange that occurs at the pulmonary alveoli. During expiration, the carbon dioxide and other related pulmonary gases are removed from the body.
Inspiration or drawing in of air is accomplished by expansion of the thoracic cavity. This is brought about by contraction of the diaphragm and intercostal muscles. The diaphragm is a modified half-dome of numerous musculofibrous tissues separating the thorax and abdomen. The diaphragm is the chief muscle of respiration. The intercostal muscles are the inner and outer layer of muscles between the ribs. These muscles draw adjacent ribs together and act to increase the volume of the thorax during inspiration.
Expiration or the expulsion of air may be active or passive. In ordinary breathing it is passive, through inspiratory muscle relaxation, wherein little muscular effort is needed to bring the chest wall back to normal position. In forced or labored respiration, muscular effort is involved.
If inspiration is accomplished chiefly by contraction of the diaphragm, the abdomen will bulge with each inspiration because the diaphragm, forming at once the floor of the thorax and the roof of the abdominal cavity, is dome-shaped with downwardly directed concavity. In contracting, it pushes the abdominal viscera down. This type of respiration is called diaphragmatic or abdominal. An opposite form of respiration is the thoracic type, in which the ribs and sternum are raised by intercostal muscle contraction.
A respiratory cycle has inspiratory and expiratory phases. At rest the former lasts about one, the latter about three seconds. Increase of thoracic capacity reduces intrapleural pressure and thereby expands the lungs, decreases intrapulmonary pressure and draws air into the respiratory passages. The expiratory phase is largely passive, wherein recoil of the thoracic wall and lungs raises intrathoracic pressure to expel air.
The muscle movements of respiration are generally controlled by the phrenic and intercostal nerves. The ipsilateral phrenic nerve arises in the cervical plexus, enters the thorax and passes to the diaphragm. In essence the phrenic nerve is the motor nerve of the diaphragm.
Previously, in patients suffering from respiratory paralysis from injury of the spinal cord at the cervical level, various attempts have been made to produce artificial respiration. In this regard, electrical stimulation of the phrenic nerves provides a useful tool by which mechanical and contractile performance of the diaphragm can be assessed (Marshall, R. Relationship between stimulus and work of breathing at different lung volumes. J. Appl. Physiol. 1962; 17:917-21; Pengelly I. D., Alderson M. A., Milic-Emili J. Mechanics of the diaphragm. J. Appl. Physiol. 1971; 30:797-805; Aubier M. Farkas G., DeTroyer A., Mozes R., Roussos C. Detection of diaphragmatic fatigue in man by phrenic stimulation. J. Appl. Physiol. 1981; 50:538-44; Danon J., Druz W. S., Goldberg H. B., Sharp J. T. Function of the isolated paced diaphragm and the cervical accessory muscles in C1 quadriplegics. Am. Rev. Respir. Dis. 1979; 119:909-18; DeTroyer A., Sampson M., Sigrist S., Macklem P. T. Action of the costal and crural parts of the diaphragm on the rib cage in dogs. J. Appl. Physiol. 1982; 53-30-9; DiMarco A. F., Nochomovitz M. L., Altose M. D., Kelsen S. G. Effects of aminophylline on diaphragm and cardiac contractility. Am. Rev. Respir. Dis. 1985; 132:800-5).
Further, phrenic nerve stimulation is also a clinically useful method of supporting ventilation in patients with high cervical spinal cord transection (Glenn W. W. l., Holcomb W. F., Shaw R. K., Hogan J. F., Holschuk K. R. Long term ventilatory support by diaphragm packing in quadriplegia. Ann. Surg. 1976; 183:566-77; Glenn W. W. L. The treatment of respiratory paralysis by diaphragm pacing. Ann. Thorac Surg. 1980; 30:106-9; Nochomovitz M. I. treated with nocturnal electrophrenic respiration. Am. Rev. Respir. Dis. 1978; 117:165-72), and irregular breathing patterns during sleep (Bradley R. D., Day A., Hyland RH, et al. Chronic ventilatory failure caused by abnormal respiratory pattern generation during sleep. Am. Rev. Respir. Dis. 1984; 130:678-80).
Unfortunately, activation of the diaphragm alone provides only limited tidal volumes and results in inward displacement of the rib cage (Nochomovitz M. L., DiMarco A. F., Mortimer J. T., Cherniack N. S. Diaphragm activation with intramuscular stimulation in dogs. Am. Rev. Respir. Dis. 1983; 127:325-9). The lack of an accurate and reproducible method of eliciting coordinated contraction of the inspiratory intercostal muscles has impeded a comparable evaluation of intercostal muscle mechanics and precluded development of clinical intercostal muscle pacing.
In certain situations phrenic nerve pacing alone has been used to ventilate humans. However, some patients with ventilator dependent quadriplegia do not have sufficient phrenic nerve function to maintain artificial ventilation by conventional phrenic nerve pacing techniques. While intercostal pacing via spinal cord stimulation (SCS) results in substantial inspired volumes (V) as indicated above, this method alone is not always sufficient to maintain artificial ventilation.